As a conventional conditional access apparatus, a receiver for digital broadcasting is mentioned, for example.
An encryption method for use in digital broadcasting in Japan is defined by the technical specification “ARIB STD-B25” regarding a content protection method of ARIB (Association of Radio Industries and Businesses).
In this technical specification, it is defined that media data and common information (ECM: Entitlement Control Message) which are encrypted are transmitted from a broadcasting station to receivers, and individual information (EMM: Entitlement Management Message) which is used when decrypting the encrypted common information is also transmitted from the broadcasting station to receivers after being encrypted.
Therefore, receivers cannot decrypt the encrypted media data until they receive the common information ECM and the individual information EMM from the broadcasting station.
FIG. 9 is an explanatory drawing showing an exchange of information between a broadcasting station and a receiver which is defined in the technical specification “ARIB STD-B25”.
When a program content (e.g., media data such as video and audio) is transmitted from the broadcasting station to the receiver, the program content is encrypted by using a scramble key Ks.
This scramble key Ks is encrypted by using another work key Kw, and the encrypted scramble key Ks is transmitted to the receiver as common information ECM which is a part of control data.
Furthermore, the work key Kw is encrypted by using another device key Kd, and the encrypted work key Kw is transmitted to the receiver as individual information EMM which is a part of the control data.
As mentioned above, the program content, the common information ECM, and the individual information EMM are transmitted from the broadcasting station to the receiver while the device key Kd is held in common by the broadcasting station and the receiver.
The receiver decrypts the desired media data encrypted by receiving the common information ECM and the individual information EMM from the broadcasting station first, acquiring the work key Kw from the individual information EMM by using the device key Kd which the receiver itself holds, and further acquiring the scramble key Ks from the common information ECM by using the work key Kw.
A filter condition for acquiring the common information ECM from the stream is common to all the receivers. For example, the filter condition is a packet ID, a table ID, or the like.
On the other hand, a filter condition for acquiring the individual information EMM from the stream is set for viewing and listening in such a way that the filter condition is changed for each specific receiver or each receiver of a specific group. For example, the filter condition is the card ID of an IC card, the identification information of a receiver, or the like.
Therefore, by simply transmitting the individual information EMM meeting only a filter condition which is set for viewing and listening in some receivers, the broadcasting station makes it possible to provide a program content which users can view and listen to in only some receivers.
The broadcasting station transmits the individual information EMM to the receivers periodically, and, when the types of individual information EMM increase, there is generally a tendency for the retransmission cycle of each individual information EMM to become long.
In each receiver, when the retransmission cycle of each individual information EMM becomes long, the length of time required to acquire the individual information EMM meeting the filter condition set up beforehand becomes long, and therefore it takes much time to acquire the work key Kw.
Therefore, the broadcasting station which is the sending source of each individual information EMM generally transmits each individual information EMM at a sufficiently preceding time. However, when, for example, the receiver is not turned on, the receiver cannot receive each individual information EMM in advance to acquire the work key Kw.
Furthermore, in order to enable the receiver to acquire the work key Kw in advance, there can be considered a method of making the receiver always operate or starting the receiver at regular intervals also when the user is not viewing and listening to a program content. However, in a case in which, for example, the receiver is mounted in a vehicle, the electric power which the receiver can use has a limitation, and there may be a case in which the receiver cannot receive radio waves satisfactorily depending on a place where the vehicle is stationary. Therefore, the above-mentioned method is not in practical.
In the following patent reference 1, a method of providing each of a plurality of home-use conditional access apparatuses each of which requires identical individual information EMM with a communications means, and, when one of the conditional access apparatuses receives the individual information EMM, informing the individual information EMM to the other conditional access apparatuses to enable the conditional access apparatuses to complement one another is disclosed.
However, each of these conditional access apparatuses is based on the premise that these conditional access apparatuses belong the same group in the home, for example, and they can complement one another to share the individual information EMM which each conditional access apparatus has acquired with the same filter condition, but cannot complement with any other conditional access apparatus having a different filter condition.
Furthermore, because the individual information EMM is encrypted typically by using another key, it is necessary to decrypt the encrypted individual information EMM as occasion demands in order for the conditional access apparatuses to complement one another to share the individual information EMM.    [Patent reference 1] JP, 2001-298722, A (see paragraphs having numbers [0026] and [0027], and FIG. 1)
Because the conventional conditional access apparatus is configured as mentioned above, a problem is that when a filter condition set to the conditional access apparatus differs from that set to another conditional access apparatus, the conditional access apparatus cannot acquire individual information EMM beforehand from the other conditional access apparatus, and therefore cannot acquire a work key Kw until the individual information EMM is transmitted thereto from a broadcasting station.
The present invention is made in order to solve the above-mentioned problem, and it is therefore an object of the present invention to provide a conditional access apparatus that can provide key information in advance to another conditional access apparatus even when a filter condition set to the conditional access apparatus differs from that set to the other conditional access apparatus.
It is another object of the present invention to provide a conditional access apparatus that can acquire key information from another conditional access apparatus beforehand even when a filter condition set to the conditional access apparatus differs from that set to the other conditional access apparatus.